Patent GB-1428419-A (GB)
Title: Safety device for a nuclear reactor
IPC: G21C9/027
Filed: 1973-05-31
Abstract: 1428419 Nuclear reactor safety apparatus GROUPEMENT POUR LES ACTIVITIES ATOMIQUES ET AVANCEES 31 May 1973 [2 June 1972] 26129/73 Heading G6C A nuclear reactor safety apparatus embodying a powdered neutron absorber comprises a reservoir 11 (Fig. 2) divided into lower, intermediate and upper compartments 15, 15<SP>1</SP> and 15<SP>11</SP> of which the lower is situated in the reactor core, and communicates with the space above the core via a tube 16, the compartments being in communication so that by pressurizing and depressurizing the lower compartment, the absorber powder, e.g. boron carbide, is caused to move between the lower and intermediate compartments and vice versa. The top end of tube 16 carries an axially movable bell 17 with a projecting stem 32. A cylindrical member 22 fitted with a seal 21 surrounds the bell 17 with clearance. Situated in a casing 8 (Fig. 3) above the reservoir 11 is a movable piston 23 with upper and lower ball valves 33, 31 and having inner and outer tubes 24, 9. The space above piston 8 can be pressurized by operation of a valve 28 and lapidly depressurized by means of electromagnetic valve 27. In operation to arm the apparatus for use, the valve 28 is opened to admit gas to the casing 8 to cause the piston 23 to descend; gas also enters the interior of tube 24 via valve 33, valve 31 being closed. Towards the end of its downward stroke, the lower end of tube 24 enters the clearance between the cylinder 22 and bell 17 and the stem 32 contacts ball 31<SP>1</SP> to open valve 31 as a result of which gas passes from the interior of tube 24 through the passage defined between bell 17 and tube 16 to expel liquid sodium coolant from the reservoir 11. At the end of its stroke, the lower end of tube 24 seals with seal 21 and a part 25 acts on bell 17. Gas in the tube 16 fluidizes the boron carbide lying in the conduit 18 around the lower compartment 15 and causes it to move up into intermediate compartment 15<SP>1</SP>, the gas collecting in upper compartment 15<SP>11</SP>. In compartment 15<SP>1</SP>, the boron carbide lies on a helical blade 20; similar blades 20<SP>1</SP>, 20<SP>11</SP> are provided in the conduit 18. The apparatus is now armed. When the apparatus is required to operate, valve 27 opens releasing the pressure in casing 8. Back-pressure in tube 24 causes valve 33 to close. The combined action of a spring 30 and gas pressure in tube 16 acting on bell 17, and hence the part 25, urges the piston 23 upwards and valve 31 closes trapping gas in the tube 24. Depressurization of the lower compartment 15 as the piston 23 is removed, coupled with the pressure in upper compartment 15<SP>11</SP>, forces the boron carbide from the intermediate compartment 151 (above the core) into the lower compartment 15 (in the core) to reduce core reactivity. Residual pressure in the tube 16 urges the bell 17 upwards so that a seal 17<SP>1</SP> mates with the flared upper end of the tube 16 to prevent re-entry of sodium into the reservoir. The apparatus can be re-armed by repressurizing the space above piston 23 to cause the latter to descend.